1. Field of the Invention
The present invention relates to a semiconductor laser device in which a light-exit end facet is coated with a reflectance control layer.
2. Description of the Related Art
In the conventional semiconductor laser devices, the temperature rise at a resonator facet during a high output power operation is considered to be a limiting factor of the maximum optical output, since a current which does not contribute to light emission is generated due to recombination at the surface of semiconductor layers corresponding to the resonator end facet.
In order to solve the above problem, coating of the resonator facet with a group III nitride having high thermal conductivity is proposed. However, researchers belonging to the present assignee have found that the thermal conductivity is seriously decreases due to oxygen capture by the group III nitride, and therefore the coating of the group III nitride does not function as desired.
The following patent publications disclose examples of coatings of the resonator facets.
(i) Japanese Unexamined Patent Publication, No. 9(1997)-162496 discloses formation of nitrides of Al, Ga, Ti, or Si coating having a thickness of 0.5 to 10 nm on resonator facets. However, the characteristics and reliability of the semiconductor laser device are greatly affected by the oxygen concentration in the nitrides.
(ii) U.S. Pat. No. 4,656,638 discloses formation of a metal layer having a thickness of 10 nm or less on a light-exit end facet of a semiconductor laser device, and formation of a reflectance control layer, made of an oxide such as Al2O3, on the metal layer, where the metal layer is made of, for example, Al, Si, Ta, V, Sb, Mn, Cr, or Ti. However, the characteristics of the semiconductor laser device deteriorate, and the reliability of the semiconductor laser device decreases, due to oxidation of the metal layer with oxygen which is externally diffused from the semiconductor layers, although the semiconductor surface is passivated by the external diffusion of oxygen.
(iii) Japanese Unexamined Patent Publication, No. 11(1999)-121877 discloses formation of a passivated layer such as a Si layer after removal of oxide from a semiconductor surface. However, the characteristics of the laser are greatly affected by the amount of oxygen in the passivated layer.
As mentioned above, conventionally, there is no effective solution which realizes control of the degree of oxidation at the boundary of the semiconductor layers, raises the COD (catastrophic optical damage) level, and increases reliability of the semiconductor laser device.
The object of the present invention is to provide a reliable semiconductor laser device in which the degree of oxidation at the light-exit end facet is appropriately controlled.
According to the present invention, there is provided a semiconductor laser device comprising: a multilayered structure being formed of a plurality of semiconductor layers made of a plurality of group III-V compounds, and having a pair of opposite light-exit end facets; and a reflectance control layer being formed on at least one of the pair of light-exit end facets, and having at least two sublayers. In the semiconductor laser device, a region near a boundary between the multilayered structure and one of the at least two sublayers closest to the semiconductor layers of the reflectance control layer of has an oxygen concentration of 15 atomic percent (atm %) or below, where the xe2x80x9catomic percent (atm %)xe2x80x9d is a percentage of atoms of an element of interest.
In order to realize a reliable semiconductor laser device, the relationship between the oxygen concentration at the boundary of the semiconductor layers and the characteristics of the semiconductor laser device must be clarified. However, conventionally, the oxygen concentration in the region near the boundary between the multilayered structure and the reflectance control layer has not been clarified on the atomic percentage basis. As described in detail later, the present inventor has clarified the relationship between the oxygen concentration at the boundary of the semiconductor layers and the characteristics of the semiconductor laser device. Such a relationship can be obtained, for example, based on the chemical shift caused by oxidation of a group III element, which is detected by the X-ray photoelectron spectrometry (XPS).
As described in detail later, the above relationship obtained by the present inventor indicates that when the oxygen concentration in the region near the boundary between the multilayered structure and one of the at least two sublayers closest to the semiconductor layers of the reflectance control layer is 15 atomic percent (atm %) or below, the light-exit end facet in the semiconductor laser device according to the present invention causes only a small amount of oxidation in the semiconductor layers and a small metamorphic change in the composition of the semiconductor layers. Therefore, the performance of the reflectance control layer is not degraded. Thus, the characteristics of the semiconductor laser device can be improved, and the reliability of the semiconductor laser device can be increased.
Further, the semiconductor laser device according to the, present invention may also have one or any possible combination of the following additional features (i) to (ix).
(i) The one of the at least two sublayers closest to the semiconductor layers, i.e., the first sublayer, may be made of a group IV element.
Since the group IV element has a function of gettering oxygen, oxygen diffuses into the semiconductor layers and the second or upper sublayers of the reflectance control layer, so that the degradation of the characteristics and the decrease in reliability of the semiconductor laser device can be prevented.
(ii) In the above case (i) , the one of the at least two sublayers closest to the semiconductor layers, i.e., the first sublayer, may be made of Si, Ge, or a compound of Si and Ge.
(iii) In the above case (i) or (ii), the one of the at least two sublayers closest to the semiconductor layers, i.e., the first sublayer, may have a thickness of 0.2 to 50 nm.
In the case of (ii) and (iii) , oxygen can be further effectively gettered.
(iv) The one of the at least two sublayers closest to the semiconductor layers, i.e., the first sublayer, may be made of at least one metallic element.
(v) In the above case (iv), the one of the at least two sublayers closest to the semiconductor layers, i.e., the first sublayer, may be made of one of Al, Ti, Ta, Mn, Cr, V, and Sb, or a compound of at least two of Al, Ti, Ta, Mn, Cr, V, and Sb.
(vi) In the above case (iv) or (v), the one of the at least two sublayers closest to the semiconductor layers, i.e., the first sublayer, may have a thickness of 0.2 to 5 nm.
Even when the first sublayer is made of at least one metallic element, oxygen in the vicinity of the boundary can be further effectively gettered since the oxygen concentration in the vicinity of the boundary is maintained 15% or below. Thus, oxidation of the first sublayer made of the at least one metallic element can be prevented, and therefore the characteristics of the semiconductor laser device can be improved, and the reliability of the semiconductor laser device can be increased.
(vii) The one of the at least two sublayers closest to the semiconductor layers may be made of a nitride.
(viii) In the above case (iv), the nitrogen in the nitride may be coupled to at least one of B, Al, In, Ga, Si, Ti, and Ta.
(ix) In the above case (vii) or (viii), the one of the at least two sublayers closest to the semiconductor layers may have a thickness of 30 to 1,000 nm.
Since the nitride has a high thermal conductivity, heat generated at the boundary can be effectively dissipated. In addition, according to the present invention the oxygen concentration in the vicinity of the boundary is maintained 15% or below. Therefore, the high thermal conductivity which the nitride inherently has can be maintained, and therefore the decrease in the maximum optical output power due to the facet degradation caused by heat can be prevented.